Acoustic/ultrasonic probes used for contact inspections are wrung against and moved on test object surfaces. The wringing movement improves the signal by removing excess coupling liquid or gel from between the probe and the work surface. Probes are also moved on the surface to locate features or examine adjacent areas of the test object. The movement with force against rough, hard, and abrasive test object surfaces causes damage and premature wear of the probe contact surface. Severely worn or damaged probes require expensive repair or replacement and result in lost time.
The need for protective surfaces for ultrasonic probes and the fragile transducers within them is seen in the usage of many types of probes and probe accessories. Existing effort and means for protecting probes from damage and wear are known but present limitations of various kinds.
One kind of existing, integral protective surface is of plates of hard material such as tungsten carbide, titanium carbide cermets or aluminum oxide ceramics, which are often bonded across the face of probes to resist wear and damage. These hard wear plates plastically deform asperities on the test object surface and slide over them. However the hard wear plates must be thin relative to the ultrasonic wavelength in the material in order to maintain a good acoustic working range and broad inspection application. In addition, even hard plates eventually wear thin, particularly the perimeter of the hard plate and any portion of the probe housing near the perimeter are further subject to wearing forces greater than the central, planar surface of the plate. In addition, housing corners and brittle, thin, plate edges forcibly moved against surface protrusions may chip and break away, accelerating probe wear and damage. Bonded hard wear plates are not easily separated from other probe structures, making repair or replacement of damaged or worn parts difficult or not possible. The provided protection of hard plates is therefore limited by the wear of the plates, and the difficulty of repair or replacement.
It should be noted that wear plate is used herein to denote a thin, hard protective layer of, or attached to, a probe. The wear plate can be directly used between the transducer and the test object or between an integrally bonded protective surface of a probe and test object.
Another kind of integral, protective surface on probes is of plastic-like, acoustic impedance matching layers, which are bonded across the ultrasonic probe face to transfer acoustic energy into water, plastic or composite materials. These probes are not designed to be directly in contact with metal test objects as the relatively soft plastic face would wear out quickly when wrung against rough metal surfaces. The soft face does provide sufficient protection to the transducer for the probe to be fastened in static contact with removable accessory devices such as consumable plastic delay lines and wedges, rubber membranes or standoff assemblies which are coupled to the test object and limit probe wear. Probe efficiency is improved when the value of specific acoustic impedance of the layer, determined by the product of a materials density and ultrasonic velocity, lies between the specific acoustic impedance of the materials on either side of the layer. Plastic, water and composite materials have densities, velocities, and specific acoustic impedances that are much lower than hard materials such as carbides or ceramics. Therefore, probes designed to transmit sound into low acoustic impedance devices such as plastic delays and wedges are preferentially made with soft, plastic-like face layers. However, these integral, plastic-like, protective face layers do not provide sufficient protection to the probes. They need another measure of wear surface to be used in contact with test objects.
One kind of transducer used in highly efficient probes is a piezoelectric composite that incorporates piezoelectric ceramic in a polymer matrix to achieve desirable material characteristics. The lower acoustic impedance of the composite along with plastic-like, matching layers improves sound transmission into low impedance accessories such as plastic delay lines and wedges. The composites structure has the further advantage of reducing lateral coupling of sound energy, which makes the composites the preferred material for array probe designs including phased array probes. Particularly, array probes are generally designed with matching layers bonded to the piezoelectric composite for efficient sound transmission into plastic probe accessories. However, the plastic-like, soft-face layers are generally vulnerable to wear. In addition, array probes have high replacement cost. Therefore, array probes are not normally used for direct contact inspections and are often used with a plastic probe accessory. The use of plastic delay lines and refracting wedges, however, introduces an array of issues from increased operation complexity, to the impact of measurement accuracy and to noises introduced in the volume of the plastic accessories. Therefore, a wear plate is desirable that protects the soft faces of array and general piezoelectric composite probes, and does not adversely affect measurement of a test object.
Another type of existing protective surface is thin plastic plates or rubber like membranes, which are removable probe accessories, acoustically coupled with liquid or gel to the face of probes as protection from wear. Means such as threaded members are used to mechanically fasten the plates or membranes to the probes. When worn, the plastic plates, rubber membranes and fastening members are easily and economically replaced by the inspection operator. However, plastic and rubber like materials may not slide in a smoothly scanning motion because they plastically deform into the interstices of work surface asperities and increase friction. A further problem with thin plastic plates and flexible membranes is that they may be penetrated by relatively large and sharp, work surface protrusions and fail to adequately protect the probe. Thick layers would provide greater protection but degrade the range of inspection. In addition, in both cases of thin and thick plastic protective surfaces, delay-line multiple echoes presents problems for inspections.
Yet another kind of existing wear or protective measure is standoff assemblies, such as water boxes and carbide inserts used in a gap scanning technique. These removable accessories extend from the perimeter of probes towards work surfaces, providing a vacant volume between the probe and work surfaces. The probe and work surfaces are acoustically coupled through a liquid or gel flooding the volume between the surfaces. Liquid or gel that leaks from the volume requires frequent or continuous replenishment. Leaking fluid may need to be contained, cleaned up or re-circulated. Equipment for containment, re-circulation and replenishment may be costly and cumbersome or impractical to use at the inspection site. As with the thickness of hard plates, plastic plates, and rubber layers, the thickness of the liquid filled volume needs to be thin relative to a wavelength in the liquid to maintain a good working range. However, the standoff devices may not adequately protect probes from work surface protrusions that are larger than the standoff distance, which could often happen.
Another type of existing probe accessory serving as a wear surface is called ultrasonic delay lines, which are thick, solid blocks or enclosed liquid columns held between the probe and work surface. Sonic energy emitted from the probe, travels for a period of time within the delay line towards the work surface where it is partially reflected back from, and partially transmitted into the test object. Energy not initially reflected back from the work surface travels for a time within the test object until it is reflected by reflectors within the test object, back towards the delay line and probe. Further partial reflections and transmissions occur at the interfaces of the probe and delay line, and the delay line and work surface, causing repeated echo sequences known as delay line multiple echoes. The period of these reverberations is determined by sound speed and length of the delay line. Delay line multiple echoes that occur prior to or coincident with signal echoes from within the test object can interfere with signal interpretation and cause measurement inaccuracies. The practical inspection range is usually limited by the travel time within the delay line. Longer delays have longer delay times, but attenuation and sound beam spread increases with length. Attenuation reduces signal strength and causes frequency distortions of the signal. Beam spread reduces signal strength and causes additional noise echoes when the spreading beam reflects from the side of the delay line. Although thick delay lines and long water columns can provide good protection for probes, they limit the inspection range.
Thus given the existing types of wear protective measures for associated probes and their shortcomings of lack of sufficient protection, desirable acoustic performance and easiness of operation, an improved removable wear plate is needed to address the market voids.
Ultrasonic Testing of Materials by publisher Springer-Verlag, 4th edition, section 10.4 gives a review of ultrasonic probes (transducers) and the associated wear or protective measures of various types. FIGS. 10.30, 10.32 and 10.34 are helpful to the understanding of the existing types of probes, wear plates or protective measures and their acoustic characteristics.